1. Field of the Invention
The present invention relates to ignition devices for performing spark ignition of fuel, mounted to an internal combustion engine having characteristics of resistance to ignition.
2. Description of the Related Art
Recently, there have been developed various types of high efficient engines with high performance and low NOx combustion capable of improving fuel consumption and performing carbon dioxide reduction. Because of often having a high boost and compression with a low concentration of fuel mixture gas, such a high efficient engine has a low ignitability (or has characteristics of resistance to ignition), which is difficult to ignite, by a spark. In order to perform combustion with high efficiency in such an internal combustion engine with a low ignitability, i.e. characteristics of resistance to ignition by a spark, it is necessary to provide an ignition device having a rapid combustion speed with a superior ignitability.
A first conventional patent document, Japanese patent laid open publication No. JP 2012-99303 has disclosed an ignition system comprised of an insulation section, a central electrode, a ground electrode, a plasma jet ignition plug, a discharge power source, and an energy supply section. The insulation section has an axial hole extending along an axial direction. The central electrode is inserted and arranged in the axial hole of the insulation section so that a front end section of the central electrode is arranged in an axial direction at a rear side of the front end section of the insulation section. The ground electrode is arranged in front of the front end section of the insulation section. The central electrode and the ground electrode form a gap. The plasma jet ignition plug has a cavity section which is formed by an inner peripheral surface of the axial hole and a front end surface of the central electrode. The discharge power source supplies a voltage to the gap formed between the central electrode and the ground electrode. The energy supplying section supplies electric power to the gap. The discharge power source supplies the voltage to the gap in order to generate spark discharge. Thus, plasma is generated in the cavity section when the discharge power source supplies electric power to the cavity section.
However, because the ignition system disclosed in the first conventional patent document generates high temperature and high pressure plasma as ignition sources in the cavity section, it is difficult to avoid the central electrode and the ground electrode from being damaged and deteriorated. It is therefore difficult for such a conventional ignition system to increase durability thereof for actual use because discharging is repeated with a short time period in an internal combustion engine which uses the ignition system.
In order to avoid such a conventional problem, a second conventional patent document, Japanese patent laid open publication No. JP 2009-121406 has proposed an internal combustion engine equipped with a barrier discharge device capable of generating free radicals, preventing electrode deterioration and improving ignitability. The barrier discharge device is comprised of a first electrode, a second electrode, a dielectric body, and a barrier discharge section. The first electrode is made of conductive material mounted to a cylinder head of a cylinder. The second electrode is arranged to face to the first electrode. The dielectric body is formed on one of the first electrode and the second electrode. When a voltage is supplied between the first electrode and the second electrode, the barrier discharge section generates free radicals in fuel mixture gas in the cylinder before spontaneous ignition by barrier discharge between the dielectric body and the electrode.
The internal combustion engine disclosed in the second conventional patent document generates non-equilibrium plasma by barrier discharge, and generates free radicals in fuel mixture gas in the cylinder of the internal combustion engine before spontaneous ignition in order to improve ignitability without electrode deterioration.
However, it is difficult for the conventional barrier discharge device disclosed in the second conventional patent document to securely ignite fuel gas in the cylinder by non-equilibrium plasma during the entire operation of the internal combustion engine. In particular, in order to more reduce fuel consumption and improve ignitability, a strong gas flow is generated in the combustion chamber of the cylinder in order to forcedly mix air and fuel injected in the combustion chamber. However, the strong gas flow blows off and scatters non-equilibrium plasma in the combustion chamber of the cylinder, and it is difficult to grow flame kernel by a direct reaction between the non-equilibrium plasma and fuel mixture gas.
Still further, because the discharge chamber is formed apart from the combustion chamber to the engine head from in such a conventional barrier discharge device, it is not always to use generated whole non-equilibrium plasma with high efficiency in ignition.
Still further, because the conventional barrier discharge device has a structure in which a base section of the discharge chamber is formed by a part of a housing casing with which the center dielectric body is fixed, the discharge chamber has large cooling capability. The discharge chamber having the above structure causes energy loss. This is a problem.